When I took Compared Anatomy classes in the college (interchangeable Medicine to Arts classes), the teacher told it was because boobs remind a butt, with activating the limbic system for reproduction. Following the evolution theory, the "butt went to the front part" because humans became bipedal, so the male can see the booty in the front when they're walking.

But, for me, my theory is men like big boobs due to a massive marketing for massive boobs XD